Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ an induction heater to warm up the fixing device quickly. For example, the induction heater is disposed opposite the outer circumferential surface of a fixing roller to heat the fixing roller. The fixing roller presses against a pressing roller to form a fixing nip therebetween through which the recording medium bearing the toner image is conveyed. As the recording medium passes through the fixing nip, the fixing roller heated by the induction heater and the pressing roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.
A thermopile is disposed opposite the outer circumferential surface of the fixing roller to detect the temperature of the fixing roller based on which a heating amount of the induction heater for heating the fixing roller is adjusted. However, the thermopile raises a problem that it may detect not only infrared rays from the fixing roller but also infrared rays from the components other than the fixing roller, resulting in faulty detection of the temperature of the fixing roller. To address this problem, the thermopile is disposed opposite the fixing roller via a through-hole produced in the induction heater. Accordingly, the thermopile detects infrared rays radiated from the fixing roller and traveling through the through-hole only, thus detecting the temperature of the fixing roller precisely.
Although effective for its intended purpose, the through-hole raises another problem. That is, as the recording medium bearing the unfixed toner image is conveyed through the fixing nip, heat conducted from the fixing roller vaporizes water contained in the recording medium into steam and volatilizes wax contained in toner of the unfixed toner image. When the steam and volatilized wax move from the fixing nip into the through-hole of the induction heater, they may adhere to the interior wall of the through-hole, causing detection error of the thermopile.
Accordingly, there is a need for a technology that prevents faulty detection of the temperature of the fixing roller by the thermopile caused by the infrared rays radiated from the components other than the fixing roller and the steam and volatilized wax generated from the recording medium bearing the toner image at the fixing nip.